1. Field of the Invention
The invention relates to a mandrel rod for pipe rolling mills where a hollow billet is rolled on the mandrel rod.
2. Background Art
Mandrel rods of this type are used for the hot rolling of pipes, especially in the so-called MPM (multiple pipe mill) process, as well as in the push bank process and in continuous pipe rolling mills (so-called continuous mill trains), and the hollow billet is rolled on the mandrel rod. (A hollow billet is understood herein to mean the hollow cylindrical intermediate product during pipe production.) The rod head can be rounded or provided with a plug of the same diameter. The wall of the hollow billet is rolled out by several roller pairs that lie horizontally and vertically opposite one another and that press the hollow billet against the mandrel rod. The mandrel rod that is moved along with the hollow billet is located, during the entire rolling operation, inside of the hollow billet and is heated by its temperature and the rolling process. After the rolling operation, it is pulled out of the hollow billet and stored to cool. For the subsequent rolling operation, another already-cooled mandrel rod is used.
Mandrel rods of this type, on which the hollow billet is rolled, are to be differentiated from those mandrel rods of another type that are used only as carriers of a plug on which the hollow billet is rolled. These mandrel rods used in plug mills have a smaller diameter than the plug, thus do not come in contact with the hollow billet and thus do not have to absorb radial rolling forces.
High demands are placed on the quality of the mandrel rods of the type mentioned above with respect to their surface hardness, straightness, surface quality and absence of deformation during rolling. During the rolling operation, the mandrel rod must be able to withstand large radial compressive forces. The mandrel rods, which can have diameters of several tens of centimeters, have up to now been rolled or forged as solid rounds whose surface was treated (finished and hardened) and into which axial bore holes were drilled for a coolant. The production and processing of the round in the required quality and length (over ten meters) was costly and expensive.